SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features

: Jacobson-Galan WV, Margutti R, Kilpatrick CD, Hiramatsu D, Perets H, Khatami D, Foley RJ, Raymond J, Yoon SC, Bobrick A, Zenati Y, Galbany L, Andrews J, Brown PJ, Cartier R, Coppejans DL, Dimitriadis G, Dobson M, Hajela A, Howell DA, Kuncarayakti H, Milisavljevic D, Rahman M, Rojas-Bravo C, Sand DJ, Shepherd J, Smartt SJ, Stacey H, Stroh M, Swift JJ, Terreran G, Vinko J, Wang XF, Anderson JP, Baron EA, Berger E, Blanchard PK, Burke J, Coulter DA, DeMarchi L, DerKacy JM, Fremling C, Gomez S, Gromadzki M, Hosseinzadeh G, Kasen D, Kriskovics L, McCully C, Muller-Bravo TE, Nicholl M, Ordasi A, Pellegrino C, Piro AL, Pal A, Ren JN, Rest A, Rich RM, Sai H, Sarneczky K, Shen KJ, Short P, Siebert MR, Stauffer C, Szakats R, Zhang XH, Zhang JJ, Zhang KC

Publisher: IOP PUBLISHING LTD

: 2020

: Astrophysical Journal

: ASTROPHYSICAL JOURNAL

: ASTROPHYS J

: ARTN 166

: 898

: 2

: 39

: 0004-637X

: 1538-4357

DOI: https://doi.org/10.3847/1538-4357/ab9e66

: https://arxiv.org/abs/2005.01782

We present panchromatic observations and modeling of the Calcium-rich supernova (SN) 2019ehk in the star-forming galaxy M100 (d approximate to 16.2 Mpc) starting 10 hr after explosion and continuing for similar to 300 days. SN 2019ehk shows a double-peaked optical light curve peaking at t = 3 and 15 days. The first peak is coincident with luminous, rapidly decaying Swift-XRT-discovered X-ray emission (L-x approximate to 10(41) erg s(-1) at 3 days; L-x proportional to t(-3)), and a Shane/Kast spectral detection of narrow Ha and He II emission lines (nu approximate to 500 km s(-1)) originating from pre-existent circumstellar material (CSM). We attribute this phenomenology to radiation from shock interaction with extended, dense material surrounding the progenitor star at r < 10(15) cm and the resulting cooling emission. We calculate a total CSM mass of similar to 7 x 10(-3) M-circle dot (M-He/M-H approximate to 6) with particle density n approximate to 10(9) cm(-3). Radio observations indicate a significantly lower density n < 10(4) cm(-3) at larger radii r > (0.1-1) x 10(17) cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of t(r) = 13.4 +/- 0.210 days and a peak B-band magnitude of M-B = -15.1 +/- 0.200 mag). We find that SN 2019ehk synthesized (3.1 +/- 0.11) x 10(-2) M-circle dot of Ni-56 and ejected M-ej = (0.72 +/- 0.040) M-circle dot total with a kinetic energy E-k = (1.8 +/- 0.10) x 10(50) erg. Finally, deep HST pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (similar to 10 M-circle dot) in binaries that lost most of their He envelope or white dwarfs (WDs). The explosion and environment properties of SN 2019ehk further restrict the potential WD progenitor systems to low-mass hybrid HeCO WD+CO WD binaries.